This application is based on Japanese patent application No. 10-135415 filed on May 18, 1998, the whole contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a solid state image pickup device, and more particularly to a solid state image pickup device for taking a two-dimensional image.
b) Description of the Related Art
A charge transfer type solid state image pickup device or so-called CCD solid state image pickup device has a plurality of photoelectric conversion elements disposed on a two-dimensional plane at predetermined pitches, reads charges generated by and accumulated in the photoelectric conversion elements, and outputs image signals corresponding to the charges. A color image sensor has a plurality of photoelectric conversion elements and color filters of three primary colors, red (R), green (G) and blue (B) formed on light reception surfaces of the photoelectric conversion elements.
FIG. 8 is an enlarged plan view showing a portion of a two-dimensional color solid state image pickup device according to prior art. Referring to FIG. 8, reference numerals 1, 2 and 3 represent photoelectric conversion elements (hereinafter, also called a light reception element). The photoelectric conversion element 1 labeled as G is a green photoelectric conversion element (hereinafter called a G element) formed with a green filter. The photoelectric conversion element 2 labeled as B is a blue photoelectric conversion element (hereinafter called a B element) formed with a blue filter. The photoelectric conversion element 3 labeled as R is a red photoelectric conversion element (hereinafter called an R element) formed with a red filter. The photoelectric conversion elements 1, 2 and 3 are disposed in a two-dimensional matrix shape at positions corresponding to pixels. The two-dimensional matrix is constituted of six rows L1 to L6 and seven columns C1 to C7.
Vertical transfer elements 4 are disposed between two adjacent light reception elements along the vertical direction and transfer signal charges obtained at each light reception element upward along the column direction. The vertical transfer element 4 is provided with four transfer electrodes per one light reception element, the four transfer electrodes constituting one transfer stage. Vertical transfer elements of one column is constituted of transfer stages of one column. Four-phase transfer pulses xcfx86V1, xcfx86V2, xcfx86V3 and xcfx86V4 are applied via signal lines 8, 9, 10 and 11 to the four transfer electrodes of each light reception element.
A G signal read gate 5 shown hatched reads signal charges from each G element 1 and transfers the charges to the vertical transfer element 4. An R signal read gate 6 shown hatched reads signal charges from each R element 3 and transfers the charges to the vertical transfer element 4. A B signal read gate 7 shown hatched reads signal charges from each B element 2 and transfers the charges to the vertical transfer element 4. The transfer pulse xcfx86V3 is applied via the signal line 10 to the G. R and B read gates 5, 6 and 7 to control read timings.
A horizontal transfer unit 12 transfers, in the horizontal direction, signal charges transferred in parallel from the vertical transfer elements 4 at the columns in the unit of one line (row). The horizontal transfer unit 12 transfers signal charges in response to two-phase transfer pulses xcfx86H1 and xcfx86H2. Arrows shown in FIG. 8 indicate the transfer direction of signal charges.
The signal charges transferred by the horizontal transfer unit 12 are converted into a voltage signal by an output circuit 13 to be supplied to an external circuit as a signal OUT. The G, B, R light reception elements 1, 2 and 3, vertical transfer elements 4, read gates 5, 6 and 7, horizontal transfer unit 12 and output circuit 13 are all formed on a single semiconductor substrate SUB.
FIG. 9 is a timing chart illustrating the operation of the two-dimensional solid state image pickup device shown in FIG. 8. A pulse 16 in a signal xcfx86IC is an integration clear pulse which is used for instantly draining (resetting) all charges in the R, G and B to a drain. This drain is the semiconductor substrate SUB. The solid state image pickup device has, therefore, an overflow drain structure which is shown in FIG. 2 of Japanese Patent Publication No. 5-236354.
Referring to FIGS. 8 and 9, a pulse 19 in a signal xcfx86V3 is used for opening the read gates 5, 6 and 7 and reading signal charges in the light reception elements 1, 2 and 3 to the vertical transfer elements 4. The signal charge storage time (integration time) tRGB of each of the light reception elements 1, 2 and 3 is determined by a period from the fall timing of the clear pulse 16 to the fall timing of the read pulse 19.
Four-phase vertical transfer pulses 20, 21, 24 and 24 are supplied as signals xcfx86V1, xcfx86V2, xcfx86V3 and xcfx86V4. Signal charges are transferred by one transfer stage of each vertical transfer element in response to one vertical transfer pulse. By repeating this operation, signal charges of one horizontal line (one row) are transferred in parallel to the horizontal transfer unit 12.
The two-phase horizontal transfer pulse signals xcfx86H1 and xcfx86H2 control the transfer operation of the horizontal transfer unit 12. Signal charges of one horizontal line are transferred in the horizontal transfer unit 12 during each period of the signals xcfx86H1 and xcfx86H2 indicated by a rectangle with diagonal lines.
In the output signal OUT, a signal 26 indicated by a rectangle with diagonal lines is an output pixel signal output from the output circuit 13. Each rectangular block corresponds to a pixel signal train of one horizontal line. Pixel signals of the first row L1 are output in the order of B, G, B, G, . . . , and pixel signals in the second row L2 are output in the order of G, R, G, R . . .
Since-the signal storage time tRGB is determined by the period from the fall timing of the clear pulse 16 to the fall timing of the read pulse 19, the color signal component ratio cannot be changed. Namely, the same sensitivity is used for each of three colors.
For example, if a white balance is set by using illumination light at 3200xc2x0 K (such as an incandescent lamp) or at 5100xc2x0 K (such as a fluorescent lamp) as reference light, a proper signal component ratio of R to G at 3200xc2x0 K is different from that at 5100xc2x0 K.
The signal component ratio of R to G cannot be changed as described above so that a proper ratio for given illumination conditions cannot be obtained. Namely, the color signal component ratio is determined only by the spectral characteristics of color filters formed on photoelectric conversion elements, and a color signal ratio suitable for a color temperature of illumination light cannot be obtained.
It is an object of the present invention to provide a solid state image pickup device capable of independently and variably controlling the sensitivities of different color light reception elements (e.g., R, G and B elements), i.e., the signal storage times, so as to obtain a color signal component ratio suitable for a color temperature of illumination light which illuminates a subject.
It is another object of the invention to provide a solid state image pickup device capable of reducing an output level difference to be caused by a difference in the amounts of crosstalk from some light reception elements to other light reception elements.
According to one aspect of the present invention, there is provided a solid state image pickup device comprising: a plurality of photoelectric conversion elements formed on a two-dimensional plane for generating and accumulating electric charges corresponding to an amount of incidence light; color filters of a plurality of colors disposed on the plurality of photoelectric conversion elements; control signal generating means for independently setting for each color a charge storage time of the plurality of photoelectric conversion elements formed with color filters and generating for each color a read control signal of the plurality of photoelectric conversion elements after a lapse of each charge storage time; reading means for reading electric charges from the plurality of photoelectric conversion elements for each color in response to each read control signal for each color; transfer means for transferring electric charges read by the reading means; and output means for outputting electric charges transferred by the transfer means to an output circuit.
It is possible to adjust the sensitivities of photoelectric conversion elements of respective colors, e.g., R, G and B by controlling a relation between charge storage times (integration times) of R, G and B elements. It is therefore possible to set an optimum color signal component ratio in accordance with a color temperature of a subject.
According to another aspect of the present invention, there is provided a solid state image pickup device comprising: a plurality of photoelectric conversion elements formed on a two-dimensional plane for generating and accumulating electric charges corresponding to an amount of incidence light; color filters of a plurality of colors disposed on the plurality of photoelectric conversion elements; control signal generating means for independently setting, for each of a plurality of different groups of color filters of one color divided in correspondence with a different color of adjacent color filters in a column direction, a charge storage time of the photoelectric conversion elements of each different group, and generating a read control signal of the photoelectric conversion elements of each different group after a lapse of each charge storage time; reading means for reading electric charges from the photoelectric conversion elements of each group in response to each read control signal for each group; transfer means for transferring electric charges read by the reading means; and output means for outputting electric charges transferred by the transfer means to an output circuit.
Fixed pattern noises to be cause by a difference amount of crosstalk to be caused by different color filter layouts can be reduced by adjusting the charge storage times of different groups of photoelectric conversion elements of a certain color.